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Pulse-Code Modulation Technique for Digital Telephone

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.• 1911 NEAR EAST TECHNICAL UNIVERSI1Y ELECTRIC AND ELEC1RONIC ENGINEERING DEPARTMENT PULSE-CODE MODULATION TECHNIQUE FOR DIGITAL TELEPHONE SUPERVISOR Prof.Dr. Fahretttin SADIKOGLU MAMEDOV ACKNOWLEDGEMENTS The authors would like to thank Prof. Dr. Fahrettin M. Sadikoglu for his supervision and constructive criticism in the development of this thesis. The authors also wishes to express his deep gratitude to his par• ents for their support throughout his life. • \ TABLE OF CONTENTS ,) / INTRODUCTION . 1 CHAPTER I : THE FUNCTION BLOCK OF THE PCM SYSTEM . 4 1.1. Sampling . 6 1.2. Quantizing; . 8 1.3. Coding & Companding . 9 1.4. Electrical Representation of PCM Signals 16 1.5. Demodulation 18 CHAPTER II: PCM TRANSMISSION SYSTEM 21 2.1. First Order PCM Systems 21 2.2. First Order PCM System Recommended by CCITI Frame Structures 23 2.3. Second Order PCM systems 28 2.4. tdm Hierarchy 29 CHAPTER III: THE TELEPHONE NEIWORK 36 3.1. The Public Switched Telephone Network (PSTN) 36 3.1.1. Switching Hierarchy of North America 38 3.2. Technical Considerations In The Planning of Digital Network For Multiexchange Areas 40 3.2.1. Numbering 41 3.2.2. Transmission System 42 3.2.3. Transmission Planning 43 3.2.4. Signalling 44 3.3. Network Structure In The Rural Area 46 3.4. A Completely Digital Network 4 7 CONCLUSION 50 REFERENCES .• INTRODUCTION Pulse - code modulation (PCM)was developed in 1937 at the Paris Laboratories of American Telephone and Telegraph Company (AT&T) by Aled Reeves. Reeves conducted several succesful transmission experi• ments across the English channel using the modulation techniqe, in• cluding pulse-width modulation (PWM). pulse-amplitude modulation (PAM) and pulse-position modulation (PPM).At the time circuits involved was enormously complex & expensive. Although the significance of Reeve's experiments was acknowledged by Bell laboratories, it was no until the semiconductor industry evolved in the 1960s that PCM because more prevalent. Currently in the world PCM is the prefered method of trans• mission within the Public Swiched Telephone Network (PSTN). PCM is a methode of digital transmission of an analog signal. The motives for using PCM in the telephone network can be summarized in the following7 points: 0 Transmission quality almost independent of distance. A char• acteristic of a digital signal is its immunity to interference. Digital signals can be regenerated at intermediate points along a transmission line with• out loss of quality. This is not the case with analogue signals where not only the signal but also the noise is amplified at the intermediate am• plification points. In the digital case the repeaters have to make the simple decision as to whether an incoming pulse is a "one" or a "zero". After the decision has been taken a fresh pulse is transmitted. It is certain that a number of incoming pulses will be so distorted that they cannot be rec• ognized correctly, but this failure rate can be made as low as required. It should however be observed, that the PCM-systems used in practice and 1 specified by CCITT are not, from a transmission quality point of view bet• ter than the FDM-system specified by CCITT. 0 Time-division-multiplex. The TDM principle allows an increase in capacity on cable pairs originally used for single telephone channels. It may be feasible to introduce PCM transmission on these pairs instead of laying new cables. 0 Economy for certain links. In certain applications, especially in the junction network, PCM transmission has shown itself to be com• petitive with any other method of transmission. The length of the trans• mission links should be in the inttermediate region where normal voice frequency (VF) links tend to be too long and FDM links too short. This optimal distance interval is very dependent on local factors such as subscriber density, topology of the country etc .. and therefore var• ies widely. However, the figures in the graph can be taken as being typical for the first order PCM multiplex system. O Economy in combination with digital switching. A high proportion of the cost for PCM-systems lies in the terminal cost. The introduction of digital switching will lsubstanttally reduce this cost since switching is per• formed directly on the digital bit stream and no costly analogue/ digital conversion is needed. A combination of digital switching and transmission will therefore tend to lower the overall cost. 0 IC-technology. Developments in integrated circuit technology seem to point to favourable cost levels and a high degree of reliabilty. 0 Integration of services. As a digital medium a PCM-link can transmit not only speech but also data, telex, encoded visual information etc. A PCM-channel has a capacity of 64,000 bit/s which makes it a very powerful data channel. 2 • New transmission media. Future wide-band transmission media, such as waveguides and optical fibres are more suitable for digital than analogue transmission. The project consists of 3 chapters: Chapter 1 presents five major principles: sampling, quantizing, electrical representation of PCM signals, Coding and Companding, de• modulation for PCM systems. This theoretical material is used it be repre• sentation first order PCM system. The functional and timing diagram is system are presented. Chapter 2 starts with description 30 and 24 channels standard PCM systems rdecommended by CCIT.The more important accept of PCM modulation technique-digital multiplexing hierarchy is analysed. The eval• uation of Transmission rate for different level hierarchy are clearly cal• culated and explained. Chapter 3 includes an application PCM system in the public switched telephone network. Different types of network is considered. New method of switching is given to interconnect ceutial office through the use of interoffice trunks and tudem trunus. Outside the local area told trunus and intertool trunus are considered. The text is arrauged so that it can be used by students to study same part of the subject "Telecommunication". It can be useful for en• gineers too. 3 CHAPTER I THE FUNCTION BLOCK OF THE PCM SYSTEM The statement above gives some idea about the basic processes in pulse code modulation. Here we shall give these processes their right names. The process of choosing measuring points on the analoguee speech curve is called sampling. The measurement values are called samples. When samlpling, we take the first step towards a digital representation of the speech signal as the chosen sampling instants give us the time co• ordinates of the measuring points. The amplitudes of the samples can assume each value in the am• plitude range of the speech signal. When measuring the sample am• plitudes we have to round off for practical reasons. In the rounding-off process, or the quantizing process, all sample amplitudes between two marks on the scale will be given the same quantized value. The number of quantized samples is discrete as we have only a discrete number of marks on our scale. Each quantized sample is then represented by the number of the scale mark, i.e. we know now the coordinates on the amplitude axis of the samples. The processes of sampling and quantizing yield a digital repre• sentation of the original speech signal, but not in a form best suited to transmission over a line or radio path. Translation to a different form of signal is required. This process is known as encoding. Most often the sam• ple values are encoded to binary form, so that each sample value is repre- 4 .. sented by a group of binary elements. Typically, a quantized sample can assume one of 256 values. In binary form the sample will be represented by a group of 8 elements. This group is in the followingcalled a PCMword. For transmission purposes the binary values O and I can be taken as cor• responding to the absence and presence of an electrical pulse. On the transmission line the pulses in the PCM words will become gradually more distorted. However, as long as it is possible to distinguish between the absence and the presence of a pulse, no information loss has occurred. If the pulse train is regenerated, i.e. badly distorted pulses are replaced by fresh pulses at suitable intervals, the information can be transmitted long distances with practically no distortion at all. This is one of the advantages of digital transmission over analogue transmission; the information is contained in the existence or not of a pulse rather than in the form of the pulse. In our picture of the graph and the table this is analogous to the fact that the information in the table is not affected if the digits are badly written as long as they are legible. But if the graph is badly drawn, loss of information is inevitable. On the receiving side the PCM words are decoded. t.e. they are translated back to quantized samples. The analogue speech signal is then reconstructed by interpolation between the quantized samples. There is a small difference between the analogue speech signal on the receiving side and the corresponding signal on the transmitting side due to the rounding off of the speech samples. This difference is known as quantizing dis• tortion. The function blocks in the pulse code modulation process are shown in figure 1. 1. 5 Analogue signal Receiver Transmlsslon line PCM signal Figure 1 - 1. Pulse code modulation Function blocks 1.1. Sampling In the practical electrical meaning, to sample is to take instaneous values of the analogue signal at equal time intervals. See figure 1-2. Orlqlnal fA '~".'' 1 ~--=:;/ •o,m, Sampler , Amplitude Samples Sampled ~/J s~~~---- I I I I "'lime - Sampling Interval t, ' f • J_ Sampling rate ' t, Figure 1 - 2.
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